From 6161c94a6108ebddafe4e95c14bde4cdc3f8c01c Mon Sep 17 00:00:00 2001
From: John Newbery <john@johnnewbery.com>
Date: Fri, 8 May 2020 14:18:13 -0400
Subject: [net processing] Only send a getheaders for one block in an INV

Headers-first is the primary method of announcement on the network. If a
node fell back sending blocks by inv, it's probably for a re-org. The
final block hash provided should be the highest, so send a getheaders
and then fetch the blocks we need to catch up.

Github-Pull: #18962
Rebased-From: 746736639e6d05acdb85c866d4c605c947d4c500
---
 src/net_processing.cpp | 24 ++++++++++++++----------
 1 file changed, 14 insertions(+), 10 deletions(-)

diff --git a/src/net_processing.cpp b/src/net_processing.cpp
index 2db2619a81..2a61f84a08 100644
--- a/src/net_processing.cpp
+++ b/src/net_processing.cpp
@@ -2261,6 +2261,7 @@ bool ProcessMessage(CNode* pfrom, const std::string& msg_type, CDataStream& vRec
 
         uint32_t nFetchFlags = GetFetchFlags(pfrom);
         const auto current_time = GetTime<std::chrono::microseconds>();
+        uint256* best_block{nullptr};
 
         for (CInv &inv : vInv)
         {
@@ -2277,17 +2278,14 @@ bool ProcessMessage(CNode* pfrom, const std::string& msg_type, CDataStream& vRec
             if (inv.type == MSG_BLOCK) {
                 UpdateBlockAvailability(pfrom->GetId(), inv.hash);
                 if (!fAlreadyHave && !fImporting && !fReindex && !mapBlocksInFlight.count(inv.hash)) {
-                    // We used to request the full block here, but since headers-announcements are now the
-                    // primary method of announcement on the network, and since, in the case that a node
-                    // fell back to inv we probably have a reorg which we should get the headers for first,
-                    // we now only provide a getheaders response here. When we receive the headers, we will
-                    // then ask for the blocks we need.
-                    connman->PushMessage(pfrom, msgMaker.Make(NetMsgType::GETHEADERS, ::ChainActive().GetLocator(pindexBestHeader), inv.hash));
-                    LogPrint(BCLog::NET, "getheaders (%d) %s to peer=%d\n", pindexBestHeader->nHeight, inv.hash.ToString(), pfrom->GetId());
+                    // Headers-first is the primary method of announcement on
+                    // the network. If a node fell back to sending blocks by inv,
+                    // it's probably for a re-org. The final block hash
+                    // provided should be the highest, so send a getheaders and
+                    // then fetch the blocks we need to catch up.
+                    best_block = &inv.hash;
                 }
-            }
-            else
-            {
+            } else {
                 pfrom->AddInventoryKnown(inv);
                 if (fBlocksOnly) {
                     LogPrint(BCLog::NET, "transaction (%s) inv sent in violation of protocol, disconnecting peer=%d\n", inv.hash.ToString(), pfrom->GetId());
@@ -2298,6 +2296,12 @@ bool ProcessMessage(CNode* pfrom, const std::string& msg_type, CDataStream& vRec
                 }
             }
         }
+
+        if (best_block != nullptr) {
+            connman->PushMessage(pfrom, msgMaker.Make(NetMsgType::GETHEADERS, ::ChainActive().GetLocator(pindexBestHeader), *best_block));
+            LogPrint(BCLog::NET, "getheaders (%d) %s to peer=%d\n", pindexBestHeader->nHeight, best_block->ToString(), pfrom->GetId());
+        }
+
         return true;
     }
 
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